Residual stress measuring method

Abstract

A method capable of accurately measuring, in an object including a columnar shaft section and a tabular section projecting outward radially beyond an outer circumferential surface of the shaft section, a distribution of residual stress near a fillet surface interconnecting the shaft section and the tabular section. The method includes repeating a measurement cycle, which includes cutting the object to form a new cutting surface and measuring residual stress in a plurality of parts on the cutting surface. The cutting surface is a conical or cylindrical surface concentric with the axis of the shaft section, being through a reference position unchanged in the repeated measurement cycle. The reference position is desirably the center position of an arc of the fillet surface having the largest diameter on the central longitudinal cross-section of the object.

The invention claimed is: 
     
       1. A method of measuring residual stress, comprising repeating a measurement cycle which includes:
 cutting an object that includes a columnar shaft section and a tabular section projecting outward radially beyond an outer circumferential surface of the shaft section over an entire circumference, the object being provided with a fillet surface in a portion interconnecting the shaft section and the tabular section, to thereby form a new cutting surface which is a conical or cylindrical surface concentric with an axis of the shaft section, the conical or cylindrical surface being through a reference position which is unchanged in every repeated measurement cycle; and measuring residual stress in a plurality of parts in the cutting surface. 
 
     
     
       2. The residual stress measuring method according to  claim 1 , wherein the reference position is a center position of an arc having a largest diameter among arcs included in the fillet surface on a central longitudinal cross-section of the object. 
     
     
       3. The residual stress measuring method according to  claim 1 , wherein the measuring the residual stress in the cutting surface includes measuring residual stress in each of a plurality of parts aligned in a ridgeline direction of the conical or cylindrical surface. 
     
     
       4. The residual stress measuring method according to  claim 1 , wherein the measuring the residual stress in the cutting surface includes measuring respective components of residual stress with respect to a ridgeline direction of the conical or cylindrical surface and a tangential direction orthogonal to the ridge line. 
     
     
       5. The residual stress measuring method according to  claim 1 , wherein the measuring the residual stress on the cutting surface includes:
 placing a strain gauge onto the cutting surface; 
 slicing a small piece including the strain gauge from the cutting surface; and 
 measuring a release strain of the small piece by use of the strain gauges. 
 
     
     
       6. The residual stress measuring method according to  claim 5 , wherein the slicing the small piece includes:
 forming a groove adjacent to the strain gauge; and 
 inserting, into the groove, a tool driven to rotate around a predetermined rotational axis to perform cutting so as to cut a workpiece in a rotational radial direction orthogonal to the rotational axis, and cutting a part on a lower side of the cutting surface with the tool to cut off the part to thereby separate a surface layer of the cutting surface. 
 
     
     
       7. The residual stress measuring method according to  claim 1 , further comprising measuring residual stress of a second object equivalent to the object, wherein
 the measuring the residual stress of the second object includes: 
 cutting off the second object along two planes through an axis of a shaft section of the second object to thereby obtain a measurement piece defined by the two planes, and 
 measuring residual stress in a plurality of parts located on a straight line corresponding to a ridgeline of the conical or cylindrical surface in a cutting surface of the measurement piece. 
 
     
     
       8. The residual stress measuring method according to  claim 7 , wherein the measuring the residual stress in the plurality of parts on the cutting surface of the measurement piece includes measuring respective components of residual stress with respect to a direction corresponding to the ridge line and a direction corresponding to the normal of the conical or cylindrical surface. 
     
     
       9. The residual stress measuring method according to  claim 7 , wherein the measuring the residual stress in the plurality of parts on the cutting surface of the measurement piece includes:
 aligning and placing a plurality of second strain gauges on a straight line corresponding to the ridgeline of the conical or cylindrical surface in the cutting surface of the measurement piece; and 
 further cutting the measurement piece to release residual stress of the measurement piece and thereby measuring respective release strains by use of the second strain gauges. 
 
     
     
       10. The residual stress measuring method according to  claim 7 , further comprising measuring residual stress in a plurality of parts in a surface of the second object. 
     
     
       11. The residual stress measuring method according to  claim 10 , wherein the measuring the residual stress in the plurality of parts on the surface of the second object includes measuring respective components of residual stress with respect to a direction of a line of intersection of the surface of the second object and a plane through the axis and a direction orthogonal to the line of intersection. 
     
     
       12. The residual stress measuring method according to  claim 10 , wherein the measuring the residual stress in the plurality of parts on the surface of the second object includes a step of placing a third strain gauge onto a surface of the second object. 
     
     
       13. The residual stress measuring method according to  claim 7 , wherein the shaft section of the object and the shaft section of the second object are integrally formed. 
     
     
       14. The residual stress measuring method according to  claim 1 , wherein the cutting surface in at least one of the repeated measurement cycles is a conical surface.